Horse

I

Introduction

Horse, large land mammal notable for its speed, strength, and endurance. Horses are members of the Equidae family, which also includes zebras and asses. Like all equids, the horse is extremely well adapted to traveling long distances with great efficiency and to surviving on a diet of nutrient-poor, high-fiber grasses. The horse is an intensely social animal, forming strong associations with members of its herd and possessing a keen ability to recognize subtle social cues. These instinctive behaviors form the basis of the horse’s ability to bond with and obey a human trainer.

The horse’s influence on human history and civilization make it one of the most important domestic animals. Horses were domesticated in Eurasia around 6,000 years ago. Throughout much of human history, they have provided humans with mobility and have served in agriculture, warfare, and sport. Today domestic horses are found throughout the world, with a total population estimated at 60 million. So-called wild horses, such as those found in the American West, are actually feral animals, free-living descendants of domestic horses that escaped or were turned loose.

The wild ancestors of the modern horse evolved for millions of years in North America. They spread to other parts of the world by traveling southward to South America and by crossing land bridges that connected North America to Europe and Asia during the ice age. Horses vanished from both North and South America in a wave of extinctions that occurred near the end of the Pleistocene Epoch, about 15,000 years ago. They were not seen in the Americas again until 1494, when Italian explorer Christopher Columbus transported them on ships from Spain on his second voyage to the New World.

Przewalski's horse, named after the Russian explorer Nikolay Przhevalsky, is believed to be the only truly wild horse to survive to modern times. Przewalski’s horse probably became extinct in the wild in Mongolia in the 1960s, but a captive breeding program in Europe helped restore the population. About 1,100 horses survive today in captivity in zoos and wildlife parks. Przewalski’s horse was reintroduced to national parks in Mongolia beginning in 1992, and several hundred now live there.

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II

Physical Characteristics

As a result of deliberate breeding by humans, horses display a remarkable variation in size, body shape, and coat color. Traditionally, a horse’s size is measured at the withers—an elevated part of the spine between the neck and the back. The measurement is made in hands; one hand equals about 10 cm (4 in). Typical riding horses stand 14 to 16 hands high and weigh 400 to 500 kg (900 to 1,100 lb). The smallest horse on record, a Falabella miniature horse, stood 48 cm (19 in), or just under 5 hands, and weighed 14 kg (30 lb). The largest horse on record was a Belgian that stood 1.8 m (6 ft) tall, or 18 hands, and weighed 1,450 kg (3,200 lb).

The horse has a hairy coat and a long mane and tail. A heavy winter coat grows in the fall and sheds in the spring. Typical coat colors include black, brown, gray, cream, gold, and white. The mane and tail can be the same or different from the body color, and many variations in color can result from inherited traits that cause spotting, dilution of the basic coat colors, or a sprinkling of white hairs in the coat. Many color patterns have specific names, such as bay (brown with black mane and tail), chestnut (reddish brown with mane and tail of the same or lighter color), and palomino (gold with a creamy white mane and tail).

A horse’s head is composed of the cranium, which encloses the animal’s large, complex brain, and the face, distinguished by a long muzzle consisting of the nose and lips. The muzzle provides enough distance between the horse’s mouth and its eyes so that it can graze and watch for danger at the same time.

Horses have the largest eyes of any land mammal. The large eyes protrude from the sides of the head, enabling horses to see almost directly behind themselves, even while facing forward. Their night vision is excellent. Horses have limited color vision, which appears to be similar to one of the less common forms of color blindness in humans—they perceive red and blue, but they cannot distinguish between green and shades of gray.

Horses have powerful teeth and jaws to grind and break down plant fibers. Their teeth grow continuously as their surfaces wear down. Male horses usually have 40 teeth and females have 36. Between the front incisors and the rear molars is a gap called the diastema, where the bit is placed. Horses can close their wide nostrils against dusty winds, and they can move their large ears to detect sounds from various directions.

A horse’s head is held up by its long, flexible neck, which lets the horse reach down to the ground to feed, rise to a high vantage point to sight danger, and bite itches on the front part of its body. The horse’s body has a wide chest, which holds its enormous lungs and heart; and a muscular back, beneath which lie the horse’s internal organs for digesting food and reproducing. A horse’s long, flowing tail helps keep its hindquarters warm and is used to swish away insects.

The specialized structures of the horse’s legs make the animal a very efficient runner. What we think of as the horse’s knee is actually the equivalent of a human’s ankle, so from the knee down the leg is really a highly elongated foot. The lowest part of the foot is the tip of a single toe, which corresponds to the tip of a person’s middle toe. This large, strong toe tip is well protected by a tough, curved hoof. By “standing on its toes,” the horse has a very long leg for an animal of its size, but also a very light leg, since toes are lightweight structures, carrying a minimum of bone and tendon and no muscle at all. Like a person’s foot, a horse’s foot has a sole. In the horse, the sole includes a rubbery, V-shaped structure called the frog, which helps absorb the impact of the foot against the ground.

Many of the joints in horses’ legs are comparable to hinges that permit forward and backward motion only. This type of joint requires fewer muscles than are needed for the kind of ball-and-socket joint that occurs in the human hip, which can rotate in any direction. This yields a further savings in weight. Long, light legs allow a horse to move very efficiently. A long leg produces a long stride, and a light leg allows the horse to swing its limbs back and forth quickly with a minimal expenditure of energy. The top speed of the horse is about 70 km/h (45 mph).

A

Internal Organs

The horse has very efficient respiratory and circulatory systems that enable it to race at high speeds without running short of air. While walking, a horse consumes only 1 liter (about 0.25 gallon) of oxygen a minute, but at a racing gallop, its oxygen consumption can approach 60 liters (nearly 15 gallons) per minute. At the gallop, the horse’s head and neck move up and down in rhythm with each stride. This motion tends to squeeze and expand the lungs, so that a galloping horse automatically takes exactly one breath per stride. This mechanism ensures that the faster the horse gallops, the more air it takes in.

The horse has a single stomach and a large digestive organ called the cecum, which forms a dead-end alley at the junction of the large and small intestines. Microorganisms that live in the cecum break down cellulose, a tough substance within the walls of plant cells, making it possible for the horse to digest grasses. The cecum has a comparable role to the rumen, a specialized stomach chamber present in ruminants, or cud-chewing animals, such as cows and sheep. Horses cannot extract as much energy out of food as ruminants do, but they are able to digest food more quickly. As a result, a horse can eat more food each day than a cow of the same size. Due to this difference, horses can survive on stemmy, high-fiber roughage that would not sustain a cow.

III

Reproduction

Horses reach sexual maturity at about one and a half years. The estrous cycle in the mare—a mature female horse—typically lasts 21 days. During the first five days of the cycle, the mare is usually receptive to mating. The estrous cycle stops during winter and resumes in the spring, which is the start of the breeding season. A stallion—a mature male horse—approaching a mare in estrus engages in various courtship rituals. These include uttering nickering sounds and sniffing and licking the mare’s genital area.

The gestational period in the horse averages 11 months. Mares generally give birth to a single offspring, or on rare occasions, twins. Young horses that have not yet been weaned are called foals. Young female horses are called fillies, and young males are called colts.

Among feral horses, stallions guard a harem of mares and compete with other stallions for “ownership” of mares. A harem commonly consists of a single stallion, one to three mares, and their immature offspring. Stallions challenge one another by competing in lengthy squealing contests; often a horse that squeals the longest is able to claim the superior position without physical combat. Stallions that take over a harem from another male will often cause abortions in pregnant mares by chasing and aggressively attacking them. This allows the new stallion to immediately rebreed the mares and produce his own offspring.

To control aggressive behavior in stallions, which is closely linked to the hormone testosterone produced in the testes, horse owners usually castrate males that will not be used for breeding. A castrated male horse is called a gelding.